Magnetic field sensors can be used to detect magnetic field direction, intensity, and the location of magnetic objects. Magnetoresistive sensors can be used to produce a high performance push-pull sensor bridge with the advantages of low offset, high sensitivity, and good temperature stability. Magnetic tunnel junction (MTJ) sensors are a new type of magnetoresistive sensor which has been used recently in various applications It utilizes a magnetic multilayer and the magnetoresistance effect is called tunneling magnetoresistance (TMR), The TMR effect is related to the magnetic orientation of magnetization of the ferromagnetic layers making up the magnetic multilayer, TMR is advantageous since it is a larger magnetoresistance effect than that of commonly used technologies such as anisotropic magnetoresistance (AMR) effect, giant magnetoresistance (GMR) effect, or the Hall Effect, thereby producing a larger signal. When compared to the Hall Effect TMR has better temperature stability. TMR has a further advantage of high resistivity and therefore can have very low power consumption. To summarize, MTJ devices have higher sensitivity, lower power consumption, better linearity, wider dynamic range, and better thermal compensation characteristics than AMR, GMR, or Hall devices. Due to its high sensitivity, the resulting noise performance of TMR sensors is better. In addition MTJ materials can be fabricated using existing semiconductor processes thereby providing the capability to produce MTJ sensors with very small size.
It is common to use a push-pull sensor bridge rather than a single sensor element in order to produce a magnetic field sensor, since bridge sensors have higher sensitivity, and because a bridge configuration removes offset and suppresses thermal effects magnetoresistive This however is not ideal for manufacturing, since there are no standard methods for setting the magnetization direction of the adjacent arms. Present techniques for manufacturing magnetoresistive sensor push-pull bridges are as follows:    (1) Double Deposition: Here, two deposition processes are used to deposit GMR or TMR films which have differently oriented pinned layers onto the same wafer. This method is complex production process, depending on the specific nature of the films, if a second anneal is needed; it can significantly affect the deposition of the first film. Because two separate deposition steps are used, it is difficult to match the resistance and performance of bridge legs built form the different films, degrading the overall performance of the sensor;    (2) Multi-Chip Packaging Technology: In this process, two or more sensor chips diced from the same wafer are packed together so that their resistance and performance characteristics are well-matched, but during packaging one is rotated relative to the other by 180° in a multi-chip package, in order to produce a push-pull half bridge. This method produces reasonably well-behaved push-pull bridges with good sensitivity and fair temperature compensation. However, because of the multi-chip packaging technology there are performance and cost disadvantages. The package size large; production costs are high; alignment is difficult and it is not always possible to achieve an accurate 180° flip; If the sensor chips are not properly aligned at 180°, then the performance characteristics of the two chips may not match well. In short, although the Multi-Chip Packaging process is standard and capable of producing good push-pull sensor bridges, it brings about cost and potential performance problems;    (3) Laser Assisted Local Magnetic Annealing: In this method, GMR or MTJ wafers are initially annealed at high temperature in a strong magnetic field, which sets the magnetization of the different bridge arms in the same direction. At a later step in the process, a scanning laser beam plus reversed magnetic field is used to locally heat the wafer in the regions where the pinned layer needs to be reversed. Although it sounds easy in concept, the local laser heating method requires special equipment that is not commercially available, and development of the equipment is expensive. The process is expensive to utilize, since it requires a long time to process an entire wafer. Performance is also an issue, since it can be difficult to properly match other performance of the push and pull sensor arms that result in the process, and the consistency of the different bridge arms cannot be guaranteed.
As illustrated above, there are few good options for producing low-cost MTJ or GMR sensor bridges with good performance using standard semiconductor processes.